1. Field of the Invention
The present invention relates to a vehicle power-generation control unit controlling a vehicle generator, and a vehicle power-generation control system including the vehicle power-generation control unit.
2. Description of Related Art
A vehicle generator is for charging a vehicle battery, and supplying electric power to electrical components of a vehicle such as an engine ignition device, lighting fixtures, and so on, thorough the vehicle battery. The vehicle generator is provided with a vehicle power-generation control unit for controlling the power generation by the vehicle generator to thereby keep the battery voltage in a predetermined voltage range regardless of load variation. As a technique for controlling the vehicle generator optimally depending on the running state of the vehicle, it is known to send a control value (target voltage or duty ratio of an exciting current, for example) reflecting the running state of the vehicle from an external device (engine control unit, for example) to the vehicle power-generation control unit.
A vehicle power-generation control unit utilizing such a technique is disclosed, for example, in Japanese Patent Application Laid-open No. 11-262299. This vehicle power-generation control unit is configured to detect the kind of a control value contained in a PWM signal sent from an ECU (Engine Control Unit) on the basis of a period of the PWM signal, and to set a control variable in accordance with a duty factor of the PWM signal. This vehicle power-generation control unit makes it possible to perform a sophisticated control, because it enables using a plurality of different control values.
It is also known to provide such a power-generation control unit with capability of protecting against break or short circuit of a cable running between the power-generation control unit and the ECU, as disclosed, for example, in Japanese Patent Application Laid-open No. 2000-32680. This vehicle power-generation control unit is configured to halt its control operation if the control value sent form the ECU does not change over a predetermined time period to avoid the vehicle generator from malfunctioning due to break or short circuit of the cable.
Incidentally, the recent ECUs include a software-based processor. Since the software used in these ECUs is large-scaled, it is not easy to completely remove bug in the software. If the vehicle power-generation control unit performs its control operation in accordance with the control value sent from the ECU operating on the software containing the bug, there is a possibility that the output voltage of the vehicle generator rises exceedingly high, thereby overcharging the vehicle battery and damaging electric components, or the output voltage of the vehicle generator falls exceedingly low, thereby causing malfunction of the electric components. Conventional vehicle power-generation control units including the ones disclosed in Japanese Patent Applications Laid-open No. 11-262299 and No. 2000-32680 have a problem in that they cannot protect against the software bug.